Batteries to be used as power supplies for cranking internal combustion engines serve as power supply means for another device, such as control units. The voltage of such a battery is preferably maintained to be equal to or higher than a preset lower limit voltage in order to ensure the reliability of each control unit operating thereon and/or to prevent the battery from being deteriorated due to over discharge.
Thus, for maintaining the battery voltage at the preset lower limit voltage, it is necessary to detect, at a high accuracy, the state of a battery to be used as a power supply for restarting an internal combustion engine. This detection of the battery status contributes to safety control and stable operation for the battery.
From this viewpoint, for example, Japanese Patent Application Publication No. 2005-274214 discloses a first apparatus for calculating the internal resistance of a battery according to the amount of change in a discharge current from the battery and that of change in the voltage thereacross during the internal combustion engine being cranked; this internal resistance is used to estimate the remaining charge stored in the battery.
In addition, US Patent Application Publication No. 2007/0200567 corresponding to Japanese Patent Application Publication No. 2007-223530 discloses a second apparatus for calculating the internal resistance of a battery using linear regression calculated based on a group of pairs of sampled current and voltage values of the battery during the internal combustion engine being cranked; this internal resistance is used to estimate the voltage drop across the battery.